In the nuclear power industry, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, known as fuel assemblies. Upon being deleted to a certain level, spent fuel assemblies are removed from a reactor. At this time, the fuel assemblies not only emit extremely dangerous levels of neutrons and gamma photons (i.e., neutron and gamma radiation) but also produce considerable amounts of heat that must be dissipated.
It is necessary that the neutron and gamma radiation emitted from the spent fuel assemblies be adequately contained at all times upon being removed from the reactor. It is also necessary that the spent fuel assemblies be cooled. Because water is an excellent radiation absorber, spent fuel assemblies are typically submerged under water in a pool promptly after being removed from the reactor. The pool water also serves to cool the spent fuel assemblies by drawing the heat load away from the fuel assemblies. The water may also contain a dissolved neutron shielding substance.
The submerged fuel assemblies are typically supported in the fuel pools in a generally upright orientation in rack structures, commonly referred to as fuel racks. It is well known that neutronic interaction between fuel assemblies increases when the distance between the fuel assemblies is reduced. Thus, in order to avoid criticality (or the danger thereof) that can result from the mutual inter-reaction of adjacent fuel assemblies in the racks, it is necessary that the fuel racks support the fuel assemblies in a spaced manner that allows sufficient neutron absorbing material to exist between adjacent fuel assemblies. The neutron absorbing material can be the pool water, a structure containing a neutron absorbing material, or combinations thereof.
Fuel racks for high density storage of fuel assemblies are commonly of cellular construction with neutron absorbing plate structures (i.e., shields) placed between the cells in the form of solid sheets. The cells are usually long vertical square tubes which are open at the top through which the fuel elements are inserted. The cells are sometimes with double walls that encapsulate the neutron shield sheets to protect the neutron shield from corrosion or other deterioration resulting from contact with water.
Each fuel assembly is placed in a separate cell so that the fuel assemblies are shielded from one another. An example of a typical existing fuel rack, is described in U.S. Pat. No. 4,382,060, to Maurice Holtz et al., issued May 3, 1983, the entirety of which is hereby incorporated by reference. The Holtz rack is comprised of structural elements including elements which are hollow and cruciform in section. Each leg of the cruciform structural element includes a neutron shield therein. The free end of the legs of the cruciform structural element converge so as to have an included angle of approximately 90 degrees. The rack is comprised of such cruciform elements as well as cooperating elements which are generally T and L shaped in section.
In certain regions of the world, the fuel assemblies used in the nuclear reactors do not have a rectangular horizontal cross-section. Instead, the fuel assemblies have a horizontal cross-section that is generally hexagonal. In such instances, existing racks having cells with rectangular horizontal cross-sections are less than optimal.
Even after removal from the pool, the fuel assemblies still emit extremely dangerous neutrons (i.e., neutron radiation) and gamma photons (i.e., gamma radiation) and it is thus still imperative that these neutrons and gamma photons be contained at all times during transfer and storage. It also imperative that the residual heat emanating from the fuel assemblies be lead away and escape from the fuel assemblies. Thus, containers used to transfer and/or store fuel assemblies must not only safely enclose and absorb the radioactivity of the fuel assemblies, they must also allow for adequate cooling. In the art, there are two type of container systems used to transport and/or store fuel assemblies, canister-based systems and cask-based systems.
Generally speaking, there are two types of casks used for the transportation and/or storage of SNF, ventilated vertical overpacks (“VVOs”) and thermally conductive casks. VVOs typically utilized in conjunction with a sealable canister that is loaded with the fuel assemblies and positioned within a cavity of the VVO. Such canisters, which are often multi-purpose canisters, often contain a fuel basket for receiving the fuel assemblies. An example of a canister and basket assembly designed for use with a VVO is disclosed in U.S. Pat. No. 5,898,747 (Singh), issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference. The second type of casks are thermally conductive casks. In a typical thermally conductive cask, the fuel assemblies are loaded directly into a cavity formed by the cask body. A basket assembly is typically provided within the cavity itself to provide support for the fuel assemblies.
The fuel basket generally acts in conjunction with the cask to support the fuel in a particular pattern, minimize load transfer to the fuel, transfer heat to the cask and control criticality.